This invention relates to a variable area convergent-divergent nozzle and particularly to a variable area convergent-divergent nozzle acting as part of an attitude control system for a vertical take off or landing aircraft.
Generally such attitude control systems are formed by a plurality of downwardly acting thrusters at the extremities of the aircraft--nose, tail and wingtips.
In vertical take off and landing (VTOL) aircraft it is well known that some form of reaction attitude control system must be used in order to allow the aircraft to be manoeuvered and remain stable while hovering because the conventional flight control surfaces will not work in the hover because there is no movement of air over them.
A known type of reaction attitude control thruster is shown in FIG. 1. The thruster is supplied with high pressure air from the aircrafts gas turbine engine along a duct 1 as indicated by the arrow 2. The duct 1 ends at a convergent nozzle 3 which accelerates the air through the exit or throat of the nozzle 3. The air leaving the nozzle 3 as indicated by an arrow 8 produces an upward thrust on the nozzle 3 parallel to the arrow 8 which depends on the exit velocity, the mass of air passing through the nozzle 3 and the pressure of the air leaving the throat 4. In order to allow this generated thrust to be controlled to allow the aircraft to be manoeuvered a gate 5 is used which pivots about a pivot point 6 under the control of a servo mechanism (not shown) to open or close the throat 4 by sliding an end plate 7 of the flap 5 across the throat 4 of the convergent nozzle 3. Convergent nozzles are relatively inefficient, so in order to allow a large enough couple to be generated on the aircraft for the attitude control to be effective the end plate 7 of the flap 5 has a curved portion 7a shaped so that when the flap 5 is moved so that the curved portion 7a covers the throat 4 of the convergent nozzle 3 the air leaving the nozzle 3 is diverted in the opposite direction to its normal flow, i.e. upwards in FIG. 1 to generate a downward thrust. By arranging a pair of attitude control thrusters on opposite sides of the aircraft's centre of gravity so that one thrusts upwards and the other downward simultaneously an increased couple on the aircraft is generated. However this creates a problem because the upwardly directed air generates a downward thrust on the aircraft causing it to loose altitude which is undesirable. The main problems with thrusters of this type are, firstly that the air passing through the nozzle 3 is relatively hot and as a result when the nozzle is operating at maximum thrust with the flap 5 moved so that the plate 7 is entirely clear of the throat 4 of the convergent nozzle 3 the convergent nozzle 3 is heated by the hot compressor air while the flap 5 remains cool, as a result differential thermal expansion occurs so the flap 5 must be sized such that when the convergent nozzle 3 is at the maximum temperature of the compressor air and the flap 5 is at the minimum ambient temperature in which the aircraft will be flown the end plate 7 of the flap 5 can still be passed across the throat 4 of the convergent nozzle 3 to block the air flow. As a result of the flap 5 having to be sized for this extreme condition, when the flap 5 and convergent nozzle 3 are at the same temperature a large gap exists between the end plate 7 of the flap 5 and the throat 4 of the convergent nozzle 3 resulting in air leakage, even when the flap 5 is placed to block the flow. Such leakage is undesirable because the air passing through the attitude control thrusters is taken from the compressor stages of the gas turbine engine providing the necessary thrust to allow the aircraft to fly and so the greater the amount of air passing through the thrusters the lower the efficiency of the main gas turbine and the less thrust or power is available for flight.
A second problem is that when the plate 7 is part way across the throat 4 of the nozzle 3 to produce an intermediate level of thrust the air no longer exits the nozzle 3 in line with the nozzle axis as shown by arrow 8 but at an angle to the nozzle axis as indicated by arrow 8A due to the assymetery of the combined nozzle throat 4 and flap end plate 7. This causes a sideways thrust to be applied to the aircraft as well as a rotating couple, making the aircraft difficult to control in the hover.
It is known that convergent-divergent (con-di) nozzles which accelerate the gas running through them to supersonic speeds are intrinsically more efficient than simple convergent nozzles but it has not been possible to design a con-di nozzle suitable for use as attitude control thrusters for VTOL aircraft. This is because use of a fixed con-di nozzle together with a valve to control the airflow through it suffers from the problem that the area of the con-di nozzle will only be optimum for the air flow at one thrust level of the thruster and as a result the nozzle be inefficient at all other thrust levels, so wasting compressed air and will provide a non-linear response of thrust against airflow or valve position and it is generally undesirable to include non linear elements in aircraft control systems.
The use of known variable area con-di nozzle designs has not been attempted because the weight of such nozzles and their expense is such that the weight and cost penalties of installing them at each attitude control thruster position in a VTOL aircraft are prohibitive.